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When you don't understand exactly how your furnace works, it's easy to wonder if it's working properly. Then your utility bill goes up, or the furnace cycles on and off more often than you think it should, and you worry whether something is wrong with your furnace. Set your mind at ease by performing a simple maintenance routine to eliminate common problem causers. Next, try a handful of diagnostic observations. If everything checks out, you know your furnace is operating properly.

Strange Smells

One of the most definitive signs that something is wrong with your furnace is an unusual sound or odor. Odors are a particularly definitive signal, since your furnace shouldn't have a smell. At the beginning of the heating season, the first few furnace cycles may produce an unpleasant odor like burning dirt -- which it is, but only for that hour or two. After that, burning smells may indicate a clogged filter or dirty heating, ventilation and air conditioning ducts. Dirty burners, an overheating system or a furnace with too much pressure due to having more than 20 percent of the duct vents closed will emit a smokey odor. Most critical of all, if you notice a gas odor, or the smell of burning plastic, something is seriously wrong with your furnace. Turn off the furnace and the gas and exit the home until the furnace is fixed.

Suspicious Sounds

Sure, most furnaces create sounds, but if you've owned your furnace for any length of time, you're probably used to the normal rattles and ticks. The thermostat clicks when it activates the furnace, and the air flowing through the vents creates a distinct sound. Snapping, crackling or sizzling sounds may indicate electrical issues. High-pitched shrieking, squealing noises point to slipping blower motor belts and blower parts that require lubrication. Pilot lights rumble, with the burners off, if they need adjustment, while the same sound points to dirty gas burners with the burners on. Grinding sounds come from shot blower bearings. A humming sound may emanate from a failing blower motor or compressor motor, and a constant clicking is caused by failing relays. Any of these sounds indicate your furnace is not functioning properly.

Unusual Appearances

Often you can tell something is wrong with your furnace simply by looking at it. On the outside, a furnace covered with excessive dirt and dust isn't healthy, and it may soon exhibit larger problems, if it hasn't already. Burn marks, moisture and rust is a symptom of a furnace that isn't working properly. Getting deeper into the unit, you can look at the blower fan to see if it is clogged with dust and dirt that needs removal. Open the burner compartment, on a gas furnace, and look at the flame. A yellow flame is starved for oxygen, usually caused by dirty burners. It also creates more carbon monoxide than a blue flame.

Fresh Filters

Even before you smell your dirty filter wafting from your vents, you can check your filter to see if it's getting dirty enough to need changing or cleaning. Not maintaining your filter will quickly cause far greater problems, so a clogged filter shows a furnace that needs immediate maintenance. Depending on the media type, it may need treatment monthly to once every six months for electrostatic washable filters.

Temperature Rise Test

You can perform a useful furnace diagnostic test just like a professional, without special tools or knowledge. A heat rise test compares the temperature of the air entering your furnace (called the return air measure) to the temperature of the air leaving your furnace (the supply air temperature). Find the temperature in your room, then insert as precise of a thermometer as possible into the heat vent closest to your furnace. Hold it, without touching the vent sides, a couple of minutes. Subtract the room temperature reading from the vent temperature reading to find your furnace's temperature rise. Compare that to the recommended temperature rise, listed on the furnace label plate, to see if you are within the normal range (usually between 35 to 65 degrees Fahrenheit). If you're not, you may have a problem with your burner, blower, filter or even in the fuel delivery system.

About the Author

Karie Fay earned a Bachelor of Science in psychology with a minor in law from the University of Arkansas at Monticello. After growing up in construction and with more than 30 years in the field, she believes a girl can swing a hammer with the best of them. She enjoys "green" or innovative solutions and unusual construction.